LATE DSCNT CLRNC FROM FL430 TO FL310 REQUIRES HIGH RATE OF DSCNT. TFC AT FL310 CAUSES TCAS TA AND RA ALERTS DUE TO CLOSURE RATE.

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OUR ACFT WAS CLEARED (LATE) BY ATC TO DSND FROM 43000 FT TO A LOWER FLIGHT LEVEL. WE THEREFORE HAD A HIGH DSCNT RATE OF ABOUT 2500 FPM. DURING THE PROCESS OF LEVELING OFF; A TCAS TRAFFIC ALERT WARNING WAS ISSUED. THE CREW SPOTTED THE TRAFFIC AT 2 O'CLOCK AND BELOW. THE TCAS PREVENTIVE RA MONITOR VERT SPD WAS HEARD; WITH THE VSI RED ARC INDICATING A 500 FPM RESTRICTION; WHEN THE TWO ACFT CROSSED ON TOP OF EACH OTHER. OUR ACFT TCAS SHOWED 1000 FT; AIR TRAFFIC CTLR REPORTED A MINIMUM SEPARATION OF 1000 FT. THE OTHER ACFT REPORTED MINIMUM VERT SEPARATION OF 800 FEET. IN OUR VIEW THIS WAS PRIMARILY A CTLR MISTAKE; GIVING A LATE DSCNT CLRNC FROM 43000 FT TO 31000 FT; WITH ANOTHER ACFT OPPOSITE DIRECTION AT 30000 FT. THERE WAS; IN FACT; NO WAY WE COULD MAKE THE CROSSING RESTRICTION AND AVOID THE RA. OF COURSE; IT WOULD HAVE BEEN BETTER TO MISS THE CROSSING RESTRICTION; BUT WE GOT THE TA ONLY 5-10 SECONDS BEFORE THE RA; SO WE HAD NO WAY TO KNOW THE HIGH DSCNT RATE WOULD BECOME SUDDENLY PROBLEMATIC.