  A simple formula for the zero-temperature linear response conductance of an
interacting mesoscopic region, threaded by magnetic flux, and attached to
noninteracting single-channel leads is presented. The formula is valid for a
general interacting system exhibiting Fermi liquid properties. As an example of
the efficiency of the formula the results for the conductance of a simple
Aharonov-Bohm ring with Kondo-Fano resonance physics are presented and compared
with numerical renormalization group results.
