since 20 June 2000
I've been thinking further about this and I've worked out a model
of what might be going on. It's purely mine, so don't put any
special credence to it and I haven't tried to make it fit Maxwell's
equations or anything like that.
Consider a single straight conductor. If you pass a current through it it creates a magnetic field. If the current is varying the magnetic field also varies. The varying magnetic field induces an emf which opposes the variation of the current. (Sorry if this is teaching grandma to suck eggs, I'll get to the interesting bit in a while.) Let us, for the sake of definiteness say that the current is increasing and that (since the voltage at the end of the wire is determined by outside forces) the induced emf reduces the current. Now consider two such conductors side by side but separated by some distance. The changing magnetic field of one will induce an emf in the other. So, if they are both excited by the same external voltage the current in each is reduced by both its own self inductance and by the mutual induction from the other.
Now the interesting bit. Consider three such conductors in a row. The conductor in the middle has the back emf of its own self inductance plus the mutual induction of the both the outer wires. But the ones on the outer have a lesser back emf. They have their own self inductance, but the mutual inductance from the nearer conductor is less (because it's carrying less current because its back emf is larger) and the mutual induction from the further conductor is less just because it is further away. So if we have three conductors connected to the same varying voltage the outer two are carrying more of the current than the inner one.
We can generalise this to a large number of conductors in a bundle. Those on the outer parts of the bundle have a lower back emf (and therefore carry more current) than those in the middle simply because the outer ones don't have as many near neighbours to induce a back emf in them. This looks awfully like the skin effect. In fact, you can see that if the bundle is thick enough and the rate of change of the exciting voltage is high enough the conductors in the centre of the bundle will have a back emf nearly equal to the exciting voltage and their current will be damn all. If you take those conductors out their contribution to the back emf of the remaining conductors will be removed and the remaining ones will carry more current than before and, hey presto, you have invented the wave-guide.
Generalizing it further to the case of a solid conductor the same size as the bundle isn't obvious and I think that's where you need to bring in the big guns of Maxwell's equations and vector calculus.