On September 4, 2009, about 1755 central daylight time, a Cessna 421C, N2349S, experienced an in-flight separation of the elevator pitch trim cable during a descent for landing at Sugar Land Regional Airport (SGR), Sugar Land, Texas. The airplane was owned by TMC Aviation, Inc. and was being operated by the pilot on a 14 Code of Federal Regulations Part 91 business flight. Visual meteorological conditions prevailed and the pilot had filed and activated an instrument flight rules (IFR) flight plan. The airplane sustained minor damage. The pilot who was the only person on board was not injured. The flight originated from Santa Fe Municipal Airport (SAF), Santa Fe, New Mexico, about 1340 mountain daylight time. While the airplane was in an 800 feet per minute descent at about 180 to 190 knots the pilot noticed the elevator pitch trim was rotating continuously in the nose up position. He disconnected the autopilot and immediately heard a "popping sound". The movement of the pitch trim stopped, and the pilot could manually turn the pitch trim wheel without force or resistance. He then hand flew the airplane using spoilers, 15 degrees of flaps and exerted approximately 10 to 20 pounds of back pressure on the yoke to keep the airplane in level flight. The subsequent emergency landing at SGR was otherwise uneventful. Examination of the airplane revealed the elevator trim tab was fixed in approximately the neutral position and the pitch trim cable had fractured and separated in the area where it passes over the rearmost pulley in the tail. Cable pulleys in the area of the cable separation appeared to rotate freely on their respective bearings and did not display any significant flat spots within the cable groove. A subsequent materials laboratory examination was conducted of several components. The pitch trim cable displayed evidence of permanent curved deformation near the fracture location, with localized smearing wear and corresponding cracking of the cable wires at the approximate inner radius of the curvature. At the fracture location, some of the cable wire ends were locally curved, and splayed out. Some of the individual wires were bent "L" shape near the fracture with smearing wear and microcracking found on several wire surfaces. Several of the fractured cable wires showed evidence of fatigue fractures. Some of the worn surface areas on the cable wires near the fracture showed the presence of cadmium. One of the cadmium plated pulley guard pins showed evidence of rubbing contact with the cable. Aircraft maintenance records were available for examination for only the previous six years. Those recent records showed numerous times when the pitch trim cables were inspected and cable tension was checked. The records did not show entries for any maintenance events when the pitch trim cable was rerouted or reinstalled.