THE HELICOPTER WAS ON A FLT TO A SITE NR GUADALUPE PASS, AN AREA KNOWN FOR ERRATIC & GUSTY WNDS. AS THE PLT WAS ON AN APCH TO A HI MEADOW (ELEV APRX 5460'), THE HELICOPTER ENTERED A DSCNT & CRASHED WELL SHORT OF THE INTENDED LNDG AREA. A WITNESS RPRTD THE MAIN ROTOR RPM SLOWED & THE ROTOR BLADES CONED PRIOR TO IMPACT. THERE WAS EVIDENCE THE HELICOPTER IMPACTED IN A LVL ATTITUDE & AT A HIGH RATE OF DSCNT. AN INV REVEALED THAT A COUPLING ON THE ENG-TO-MAIN TRANSMISSION DRIVE SHAFT HAD BECOME DISCONNECTED. THERE WAS EVIDENCE THAT THE MALE DRIVESHAFT COUPLING WAS NOT PROPERLY SECURED TO THE FLANGE ON THE MAIN ROTOR DRIVESHAFT'S ENG END. EIGHT DAYS PRIOR TO THE ACDNT, AN AIR COND SYS WAS INSTALLED ON THE HELICOPTER. THIS WOULD HAVE REQD THAT A DRIVE SHAFT COUPLING BE DISCONNECTED. THERE WAS EVIDENCE THAT THE SELF LOCKING NUTS WERE REUSED WHEN THE COUPLING WAS RECONNECTED.