On August 7, 1999, at 2359 mountain daylight time, a Cessna TU206A, N4835F, sustained substantial damage when it departed the left side of runway 01 during landing roll at Animas Air Park, Durango, Colorado. The commercial pilot and his four passengers were not injured. Visual meteorological conditions prevailed for this personal flight operating under Title 14 CFR Part 91 and no flight plan was filed. The flight originated from Window Rock, Arizona, at 2100. Weather at the time was clear skies with wind from 350 degrees at four knots. According to the pilot, the aircraft had traveled a short distance following a normal touchdown on runway centerline when the aircraft pulled hard to the left and exited the left side of the runway onto a gravel covered area. He said that as the aircraft decelerated through an estimated 10 miles per hour, the nose wheel broke off. Examination of the aircraft by a mechanic employed by Greg's Flying Service, in Durango, provided evidence that the left wing, propeller, and nose landing gear has sustained damage. The mechanic also found that one of the nose wheel steering arm roll pins was missing and a piece of safety wire was substituted, and the other roll pin and roll pin receptacles were worn beyond limits. (See attached diagram.) According to the flying service mechanic, this would cause the nose wheel steering to be sloppy and slow to respond to pilot input. When the nose wheel steering had work performed on it is unknown.