Just loosen the stem polt (don't remove it), and then slightly knock it (stem polt) with hammer. Wedge/expander should come loose instantly.

But if you should really have some kind of steerer tube that's narrower further down, that I have never seen, then try removing stem polt and then try pulling handlebars out, twist and use some force if necessary. If you get it out, then screw stem polt back into wedge/expander and do circular moves with head of the stem polt to get expander loose.

Or use the method described in previous post