WHILE ON SHORT APCH TO SJC RWY 30L; THE CREW ON LGT ACR X OBSERVED SMALL FIXED WING ACFT Y ON L SIDE AT THE SAME ALT ON A CONVERGING COURSE. ACR X MADE AN EVASIVE MANEUVER TO THE R THEN A GAR TO RETURN LAND. ACR X CREW ESTIMATED SEPARATION OF ZERO FT VERT AND 300 FT HORIZ. TFC WAS NOT ISSUED TO ACR X AND TCASII DID NOT ALARM.

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ON VISUAL APCH TO RWY 30L AT SJC; WE INITIATED PULL-UP AND R BANK AFTER SEEING A LIGHT; SINGLE ENG AIRPLANE ON CONVERGING COURSE FROM L. THE OTHER ACFT WAS 300-500 FT TO OUR L; IN A SHALLOW L BANK. WE WERE PROBABLY OUTSIDE HIS FIELD OF VISION. MY FIRST REACTION WAS THAT OTHER ACFT WAS ATTEMPTING TO LAND ON OUR RWY; BUT IN RETROSPECT; PROBABLY OVERSHOT FINAL TO CLOSELY SPACED RWY 29. WE WERE IN CONTACT WITH SJC TWR AND AT NO TIME WERE WE ADVISED OF TFC IN PATTERN FOR RWY 29. WE CONTINUED TO GO AROUND AND FLEW AN UNEVENTFUL R PATTERN TO LNDG. ALTHOUGH WE HAD BEEN CLRED FOR VISUAL APCH; IT WOULD HAVE BEEN MY EXPECTATION THAT APCH CTL OR TWR WOULD ADVISE US OF POTENTIAL TFC CONFLICT ON SUCH A CLOSELY SPACED RWY. THE GAR WAS INITIATED WITH A PULL-UP AND R TURN TO AVOID COLLISION. I DID NOT IMMEDIATELY CORRECT BACK TO RWY 30L CTRLINE AS I WAS PREOCCUPIED WITH ACFT CONFIGN IN THE INITIAL STAGES OF GAR. OUR RESULTING FLT PATH TOOK US OVER RWY 30R AND COULD HAVE CAUSED A POTENTIAL CONFLICT WITH TFC LNDG/DEPARTING ON THAT RWY. NO APPARENT CONFLICT OCCURRED AS WE WERE ASSIGNED A 300 DEG HDG AND 3000 FT ALT SHORTLY AFTER THE GAR. SUPPLEMENTAL INFO FROM ACN 401050: B757 ON FINAL AHEAD OF US. WE NEVER RECEIVED A TCASII TFC ALERT ON THIS TFC. THE PNF ON THE L SIDE OF ACFT TOOK CTL OF THE AIRPLANE SINCE HE BELIEVED THE OTHER PLANE WAS ON A COLLISION COURSE WITH US. RECOMMEND REVIEW OF APCH CTL TO TWR HDOF PROCS AND TWR TFC CTL PROCS FOR PRACTICE APCHS TO SJC. RECOMMEND REVIEW OF OTHER ACFT DATA TO INCLUDE XPONDER CODES USED AND IF SQUAWK WAS ACTIVE SO TCASII SHOULD HAVE ADVISED. REQUEST FOLLOW-UP AND DISPOSITION OF THIS INCIDENT BE FORWARDED TO ALL PARTIES INVOLVED.