Ruthenium

At a twenty-nanometer pitch, the barrier is eating a punishing fraction of the wire's cross-section, and the copper left in the middle is thin enough that electrons scattering off surfaces and grain boundaries drive the resistivity up steeply.

Ruthenium wins by losing less.

Its bulk resistivity, around seven microhm centimeters, is actually worse than copper's.

But its electron mean free path is only about six point seven nanometers against copper's roughly forty, so it degrades far more gracefully as wires get narrow.