I believe it's possible but couldn't figure out the syntax. Something like this:
xmlNode.SelectNodes("//*[count(child::*) <= 1]")
but this is not correct.
Use:
//node()[not(node())]
In case only element leaf nodes are wanted (and this needs clarification -- are elements that have non-element children considered leaf nodes?), then the following XPath expression selects them:
//*[not(*)]
Both expressions above are probably the shortest that select the desired nodes (either any-node or element -- leaf nodes).