I think elevator conversation patterns are interesting. Typically in an office in the US we don't discuss anything at all on an elevator if the people are all strangers - it can be stone silence for 70 floors. However, if there is anything at all discussed (beyond "four please") - even something as trivial as the weather or how bad the traffic was getting off the loop - then usually we feel compelled to impart some sort of leave-taking expression when one of the parties leaves the elevator.