richms: It will probably do it if the device also supports it. Its off by default on several laptops I have looked at because performance is typically a lot worse than 20MHz, and some devices will just not support it at all.
It may also only enable it if the extension channel is clear, which it almost never is. Thats part of some spec for wifi but its part that everyone ignores because what is important for end users is seeing a larger number in the connection statistics because that is like signal bars - all but useless other than to make people feel good.
there is no setting at all for it