I’m not fond of the term twitchiness as much as faster or slower steering. Twitchiness makes me think of elements of design such as head angle and trail/rake.

Speaking in generalities, a longer stem technically slows the steering, while a shorter stem speeds it up. Handlebar width also effects steering as narrower speeds up and wider slows it down. But you probably will not notice small changes.

However, in real life it is dependent on the bike as I have put a flat bar on a road bike and it “felt” more lively than with the narrower road bars.

John