Just like what Sarvanan said, 40 MHz is only available on 802.11a (5GHz) and not on 802.11b/g (2.4GHz).
However, in old versions of code (6.0.182 or something like that) for wirleless controller I remember the 40 MHz was also available for 2.4 GHz (802.11b/g). They recognized later it is not feasible to add it to 2.4 GHz so they removed it from later releases.
HTH
Amjad
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