The Multimode Conductance Formula for a Closed Ring

The multimode conductance of a {\em closed} ring is found within the framework of a scattering approach. The expression can be regarded as a generalization of the Landauer formula. The treatment is essentially {\em classical} because we assume short coherence time. Our starting point is the Kubo formalism, but we also use a master equation approach for the derivation. As an example we calculate the conductance of a multimode waveguide with an attached cavity.