We have extended the circuit theory of Andreev conductance [Phys.~Rev.~Lett.
{\bf 73}, 1420 (1994)] to diffusive superconducting hybrid structures that
contain an Aharonov-Bohm ring. The electrostatic potential distribution in the
system is predicted to be flux-dependent with a period of the superconducting
flux quantum Φ0​=h/2e. When at least one tunnel barrier is present, the
conductance of the system oscillates with the same period.Comment: 4 pages RevTex including three Postscript figures. Also available at
http://www.tn.tudelft.nl/tn/thssci.htm