On March 4, 1995, at 2229 hours Pacific standard time, the crew of an American Eagle ATR-42, N266AT, operating as flight 947 from Los Angeles, California, to Las Vegas, Nevada, encountered moderate mixed icing and two associated lateral movements during the descent into Las Vegas. The flight, with a crew of 3 and 22 passengers onboard, continued on uneventfully to Las Vegas and landed at 2250 hours. The flight was in instrument meteorological conditions during the descent. After reaching lower altitudes and warmer temperatures, the crew reported the flight controls returned to normal operation. The flight crew indicated they observed 3/4 inches of mixed ice accumulate in just a few seconds on the pilot's ice detection indicator when passing through 15,500 feet. The first officer was flying the aircraft with the autopilot off. He described the flight controls as stiff and sluggish with a "buzzing" feeling, followed by a definite vibration in the ailerons and rudder controls. When passing through 14,00 feet, the crew noticed the aircraft bank about 5 degrees in both directions without their control input. The engine power was at 95 percent and the aircraft was at an indicated airspeed of about 230 knots. The aircraft was in a clean configuration and the crew selected level 3 on the anti-ice equipment. The crew also reported some ice accumulate just aft of the wing deice boots. Both pilots reported the air as smooth with no turbulence encountered while cruising at 17,000 feet, and throughout the descent to 8,000 feet. They did experience some momentary light turbulence below 8,000 feet as they neared Las Vegas after crossing a ridge line. Some of the passengers were contacted afterwards and interviewed over the telephone. One passenger remembered the flight and observed the lights on the engine and seeing snow, but doesn't recall any turbulence. Another nonrevenue passenger remembers some turbulence during the entire trip, but nothing excessive. Two other passengers described the turbulence as "too much" and a little "scary." The weather the pilots received from the dispatcher prior to the flight did not indicate any known or forecast icing conditions for their route of flight. The air traffic controller working the flight at the time indicated that he had received no reports from other pilots in the area of icing prior to the reported incident. After the report of ice, an icing notification alert was issued. The aircraft was inspected by maintenance personnel after the aircraft landed. The flight controls, flaps, and associated systems and equipment were found to be operational with no malfunctions being noted. Both pilots were trained by company personnel in ATR icing conditions procedures earlier in the year. They reported the training was adequate and pertinent. The flight data recorder was removed from the aircraft and sent to the National Transportation Safety Board laboratory for a readout. The data indicated no significant flight control deflections or indications of a control problem.