C310 PLT MISREADS ALTIMETER BY 10000 FT ARRIVING LAS.

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EVEN THOUGH CONDITIONS WERE VFR FOR THE ENTIRE RTE; I FILED AN IFR FLT PLAN FROM RENO TO BOULDER CITY ARPT; J92 BTY J86 BLD DIRECT AT FL190. I WAS CLRED TO BOULDER CITY ARPT; RENO 2 DEP; V105 OAL AS FILED. ORIGINALLY CLRED TO FL190; I WAS LATER CLRED TO FL230. SUBSEQUENTLY; I WAS CLRED TO CROSS FUZZY INTXN W OF LAS VEGAS AT 15000 FT. LATER; LAS APCH CTL CLRED ME DOWN TO 11000 FT. J86 IS A STRAIGHT LINE BTWN THE FUZZY INTXN AND THE BOULDER CITY VOR. AS I PROCEEDED ALONG THIS RTE AT 11000 FT; THE CTLR ASKED ME FOR MY HDG. WHEN I GAVE IT TO HIM; HE ASKED ME WHY I WASN'T FOLLOWING THE FUZZY ARR. I TOLD HIM I WAS CLRED J86 FROM FUZZY INTXN TO THE BOULDER CITY VOR. HE ORDERED A R TURN AND ASKED ME TO IDENT AND FOR MY ALT. I IDENTED AND TOLD HIM I WAS AT FL210. WE QUICKLY ESTABLISHED THAT I WAS AT 11000 FT. I DO NOT KNOW WHY I MISREAD THE ALTIMETER. IT COULD HAVE BEEN FATIGUE AS I HADN'T GOTTEN MUCH SLEEP THE NIGHT BEFORE OUR XA55 DEP. IT COULD HAVE BEEN BECAUSE I HAD BEEN AT THE FLT LEVELS THAT DAY AND THE DAY BEFORE AND THAT HAD BEEN IN MY MIND. WHATEVER THE REASON; IT COULD HAVE BEEN EXACERBATED BY MY BEING FLUSTERED BY THE CTLR'S ACCUSATORY TONE ABOUT MY NOT FOLLOWING AN ARR PROC FOR WHICH I HAD NOT RECEIVED A CLRNC. IN ANY EVENT; I CLRLY MADE A MISTAKE.