THE PLT HAD JUST COMPLETED A FLIGHT FROM FARMINGTON, N.M., TO PHOENIX, AZ (PHX) WHERE SHE DEPLANED HER PAX. THE PLT DEPT PHX ENROUTE TO MESA, AZ, (FFZ). AFTER T.O. THE LCL CONTROLLER INSTRUCTED THE PLT TO TURN RIGHT TO 300 DEG AND TO TURN ON THE XPNDR; THE PLT COMPLIED. SHORTLY THEREAFTER THE LCL CONTROLLER INSTRUCTED THE PLT TO CONTACT THE DEPT CONTROLLER. THE PLT DID NOT ACKNOWLEDGE OR COMPLY WITH THE INSTRUCTIONS. GND WITNESSES RPTD THAT THE AIRPLANE WAS FLYING AT A LOW ALTITUDE WHEN IT PITCHED OVER & CRASHED. THE INVESTIGATION DSCLD NO EVIDENCE OF ANY PREIMPACT AIRFRAME/ENGINE MALFUNCTIONS. THE CABIN ENTRANCE DOOR WAS FOUND TO BE OPEN AT IMPACT, BUT THE LATCHING MECHANISM WAS IN THE LOCKED POSITION. THE PLT'S POSTMORTEM EXAM DISCLOSED THAT SHE WAS NOT WEARING A SEAT BELT AT THE TIME & HER RT ARM WAS EXTENDED AT IMPACT.