THE PLT REPORTED THAT DURING ARRIVAL, HE ENTERED THE DOWNWIND WITH NO PROBLEM; HOWEVER, WHEN HE ATTEMPTED TO TURN ON A LEFT BASE, THE AILERON CONTROL LOCKED UP. HE STATED HE COULD TURN NEITHER LEFT NOR RIGHT BY USING AILERONS. REPORTEDLY, THE ACFT WOULD BANK TO THE RIGHT WHEN HE USED LEFT RUDDER & VICE VERSA. THE PLT DECLARED AN EMERGENCY. AFTER CONTINUING WESTERLY APRX 6 MI, HE REDUCED POWER & LANDED IN A WHEAT FLD. DURING HE LANDING ROLL, THE NOSE GEAR HIT A DITCH & COLLAPSED AND THE ACFT NOSED OVER. AN EXAM OF THE FLT CONTROLS FAILED TO DISCLOSE ANY PREIMPACT FAILURE OR MALFUNCTION. A CHECK OF THE AUTO-PILOT REVEALED THAT THE CONTROL SERVO, PN 1C-465-1-732, WAS COVERED WITH AN OILY APPEARING SUBSTANCE THAT WAS STICKY, AND WOULD NOT SLIP UNTIL WELL ABOVE THE SPECIFIED 42 LBS OF CABLE TENSION. NO OTHER PREIMPACT MALFUNCTIONS OR FAILURES OF THE ACFT WERE FOUND. THE RELATIVELY INEXPERIENCED PLT DID NOT REMEMBER ENGAGING THE AUTO-PILOT.