No, one would not [expect can not], because it is pronounced as one word.
It seems to me that the pronunciation is the only reason for writing it as one word. The supposed ambiguity between “can not” (implying “has a choice") and “cannot” (implying “has no choice") looks like a red herring to me. Otherwise we would write “couldnot”, since “could not” might be thought equally ambiguous. The same applies to “may not”.
In terms of pronunciation, I think that what distinguishes “cannot” is the shortness of its syllables (short a, short o, and only a single consonant sound in between).