What was found
(CAN) ON APPROACH TO A LEASE SITE THE ENGINE QUIT AND PILOT HAD TO AUTOROTATED INTO CUTLINE. ENGINEER REPLACED MAGNETO. SUPPLEMENTAL INFORMATION: "SYMPTOMS APPEARED ON SET UP FOR FINAL APPROACH APPROX 300-400 FT AGL, ENGINE RPM DROPPED SIGNIFICANTLY FOLLOWED BY LOW ROTOR HORN. PILOT SUSPECTED AN ENGINE FAILURE AND ENTERED AUTOROTATION. ENGINE RPM CAME BACK UP MOMENTARILY DURING FLARE BUT DROPPED AGAIN DURING TOUCH DOWN. ENGINE REMAINED RUNNING AFTER TOUGH DOWN. AFTER CONSULTATION WITH MAINTENANCE, THE AIRCRAFT WAS STARTED AND HOVER TESTED. IT WAS DETERMINED THAT THE GOVERNOR SYSTEM HAD FAILED. THE L/H MAGNETO THAT SENDS THE SIGNAL TO THE GOVERNOR CONTROL SYSTEM WAS REPLACED.