Mercury-Arc Valve

Solid metals (and liquid metals) tend to have a characteristic bulk conductivity that can be reliably measured in a conventional way.

Then with things like wiring, the thicker the wire, the more current can be conducted through it, and the thinner the wire, the less current.

When it comes to gaseous metal, that stuff can be really thin.

Its conductivity might just drop low enough to be in the semi- category.