ACR X NON ADHERENCE TO ATC CLRNC. ALTDEV OVERSHOOT OF XING RESTR.

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I WAS FLYING AS FO FROM PHX TO ONT AND WAS THE PF ON THIS LEG. ZLA GAVE US AN ASSIGNMENT TO DSND AND MAINTAIN 14000 FT. DURING THE DSCNT WE WERE SWITCHED TO ONT APCH AND INSTRUCTED TO CROSS AND INTERCEPT AT 14000 FT; THEN DSND AND MAINTAIN 6000 FT. IT IS OUR NORMAL PROC TO ALLOW THE AUTOPLT TO CAPTURE ANY ASSIGNED ALT; AND ONCE THIS IS ACCOMPLISHED MOVE THE ALT CTLR TO THE NEXT ASSIGNED ALT. IN THIS CASE THE CAPT MOVED THE CTLR TO 6000 FT. BECAUSE OF THIS; AND ALSO BECAUSE I DIDN'T INTERPRET THE CLRNC PROPERLY; I THOUGHT I WAS TO CROSS AN INTXN AT OR BELOW 14000 FT AND MAINTAIN 6000 FT. I WAS NEARING THE INTXN AND WOULD EASILY MAKE THE RESTR BUT ELECTED TO CONTINUE THE DSCNT DOWN TOWARDS 6000 FT. I WAS THEN NOTIFIED BY APCH CTL THAT I HAD DSNDED BELOW THE ALT ASSIGNED TO CROSS THE FIX. I FEEL THAT THE PROB AROSE PRIMARILY OUT OF A BREAKDOWN IN CREW COORD AND RESOURCE MGMNT. I ALSO FEEL FATIGUE MAY HAVE PLAYED A SMALL ROLL IN THIS OCCURRENCE. I WOULD SUGGEST THAT NASA AND OTHER SAFETY ORGANIZATIONS CONTINUE TO STRESS THE IMPORTANCE OF COCKPIT RESOURCE MGMNT IN MULTI-PLT ACFT.