On June 7, 1998, at 1230 hours mountain standard time, an Arner Challenger II S, N2NX, an experimental aircraft, experienced a loss of engine power during cruise and collapsed the landing gear after making an emergency landing in a dry wash near Pleasant Valley, Arizona. The aircraft, operated under the provisions of 14 CFR Part 91, sustained substantial damage. The private pilot/owner and one passenger were not injured. Visual meteorological conditions existed for the personal flight and no flight plan was filed. The flight originated from the Pleasant Valley airport at 1215. The pilot reported that they had departed the Pleasant Valley airport approximately 15 minutes before the loss of power was experienced. Approximately 6 miles south of the airport, the pilot noted a loss of rpm's and could not maintain level fight. He stated that he chose a dry wash area to make the emergency landing. After touchdown the aircraft sank into the sand and came to an abrupt stop. The pilot conducted an engine inspection on-scene. He observed a broken spark plug retainer spring and the detachment of the spark plug wire cap associated with it.