  Currents in a metallic ring with a quantum dot are studied in the framework
of a Langevin equation for a magnetic flux passing through the ring. Two
scenarios are considered: one in which thermal fluctuations of the dissipative
part of the current are modelled by classical Johnson-Nyquist noise and one in
which quantum character of thermal fluctuations is taken into account in terms
of a quantum Smoluchowski equation. The impact of the amplitude and phase of
the transmission coefficient of the electron through a quantum dot on current
characteristics is analyzed. In tailored parameter regimes, both scenarios can
exhibit the transition from para-- to diamagnetic response of the ring current
versus external magnetic flux.
