A landing aircraft was instructed to back taxi on parallel Runway 30R after landing but in fact turned around on Runway 30L causing a go-around event at SJC.

Narrative

Late in the evening I was working all positions combined at SJC. Numerous aircraft were pushing back off gates; calling for clearances; asking for taxi instructions; and establishing themselves on final approach. A CRJ landed on Runway 30L with another aircraft; Aircraft Y; on a two mile final. As my main taxiways were blocked with aircraft pushed back off gates; I instructed the CRJ to 'back-taxi on Runway 30R; exit at Taxiway Foxtrot; and taxi to the ramp on my frequency.' The pilot read back the instructions correctly; and I watched the aircraft make a right turn to clear Runway 30L. I then scanned my radarscope for other aircraft; looked up; and saw Aircraft Y executing a missed approach. I instructed Aircraft Y to 'fly runway heading; maintain 3000 FT.' Aircraft Y complied and as I scanned the runway; I saw lights on Runway 30L. I asked; 'CRJ; are you still on Runway 30L?' The pilot replied; 'Affirmative. We thought we were supposed to back-taxi on this runway down to Taxiway Foxtrot.' There was no loss of separation as Aircraft Y executed a missed approach before making contact with the runway.