"wanna" is a lexical item, not just a contraction

I was reading The Analytic/Synthetic Distinction (Georges Rey) [REVISED: August 15, 2008] and came across this:

"An ungrammatical sentence like “Bill is the man I wanna take a walk” might suffice on occasion for thought and communication (of “Bill is the man who I want to take a walk,”), but it's a striking fact that speakers of English—even four-year old ones!—nevertheless find it problematic (see Crain and Lillo-Martin 1999)."