THE ACFT COLLIDED BELOW THE CREST OF AN 8100 FT (MSL) MTN ON A FLT FROM MAMMOTH-JUNE LAKES ARPT (MMH), MAMMOTH, CA, TO RENO, NV, WHILE EXECUTING A LEFT TURN COURSE REVERSAL. THERE WERE NO WITNESSES TO THE ACCIDENT. WITNESSES REPORTED SEEING THE ACFT FLYING AT ABOUT 100 AGL ALONG HWY 395. THE WITNESSES ALSO RPTD LOW CEILING & VIS PREVAILED. THE PLT OBTAINED A WX BRIEFING PRIOR TO DEPARTING MMH & WAS INFORMED THAT THE MTN TOPS WERE OBSCURED BY LOW CLOUDS & SNOW SHOWERALONG THE PROPOSED ROUTE OF FLT. THE PLT STATED HE WOULD WAIT AWHILE & DEPART. HE WOULD RETURN TO MMH IF HE COULD NOT NEGOTIATE THE HIGH MTNS. EXAMINATION OF THE ACFT WRECKAGE DISCLOSED NO EVIDENCE OF ANY PREEXISTING MALFUNCTIONS OR FAILURES. THE ACFT WAS DESTROYED BY IMPACT/POST-IMPACT FIRE.THE PROP ASS'Y DISCLOSED EVIDENCE OF POWER BEING DEVELOPED AT THE TIME OF IMPACT.