What are examples of natural monopolies that don't have overwhelming economies of scale? On wikipedia, I read that the currently-used formal definition of a natural monopoly is where “[a]n industry in which multi-firm production is more costly than production by a monopoly”.

I tried looking up things related to monopolies and subadditivity, but couldn't find anything that made it easy to understand.

To further quote wikipedia, "Baumol also noted that for a firm producing a single product, scale economies were a sufficient but not a necessary condition to prove subadditivity."

Note that in the above quote, I'm almost certain the "scale economies" they're talking about are cases where the optimum scale is large enough to serve the entire market. 

So my question is, what is another set of conditions that don't involve "scale economies" that is a sufficient condition to prove subadditivity, and thus show that a situation is a natural monopoly?
One answer I found are natural monopolies that form on the basis of significant network effects. While economies of scale decrease the cost per unit as you can produce more units, network effects increase the value per unit as you sell more units. These two things together seem like they cover everything, but I haven't read any sources that mention whether there are other situations than economies of scale and network effects. Btw, I'm including "economies of scope" in my mention of economies of scale, since they're so similar.