(Sheridan, OR)  --  Police are investigating a crash that killed a woman in a wheelchair in Yamhill County.  Oregon State Police say the wreck took place yesterday morning on Highway 18B in Sheridan, leaving 66-year-old Connie Nickeson dead.  Troopers say the 81-year-old driver left the scene of the accident, but was later tracked down and cooperated with the investigation.  Authorities say it appears impairment was not a factor in the crash.