The pilot/builder of the experimental amateur-built, tailwheel-equipped airplane had just completed a flight as part of phase one flight testing. He reported an uneventful approach to landing, with the main landing gear touching down first. However, as soon as the tailwheel made contact with the runway, the airplane yawed violently to the right. The pilot applied full left rudder and brake control input, but the airplane did not respond, and departed the runway descending down an embankment. The airplane sustained substantial damage to the upper wing spar and fuselage structure at the landing gear attach points. Postaccident examination revealed that the tailwheel was locked 90 degrees to the right. The tailwheel assembly was disassembled, and the steering arm cam spring appeared jammed between the shaft and housing. The inner surface of the housing exhibited scoring marks, consistent with a pre-existing failure of the spring. The pilot reported that he had purchased the tailwheel assembly used, and that he did not have any records indicating its manufacturer or serial number.