Set short preamble??

[chkuo]

I try to set hostap-0.0.2 to short preamble as 

     "iwpriv wlan0 set_rid_word 64688 2"

the 64688 is 0xFCB0(cnfShortPreamble)

but in my cisco 350 card, it still show 

	Using Short Radio Headers: No

I think it is still LONG preamble.
So what should i do now?
And can sniffer or any tools can get short preamble?
(i try to use the NAI Sniffer,but i can just see the MAC layer and upper)
I would appreciate your any information,thank you.