  The dephasing time in coupled mesoscopic conductors is caused by the
fluctuations of the dipolar charge permitted by the long range Coulomb
interaction. We relate the phase breaking time to elementary transport
coefficients which describe the dynamics of this dipole: the capacitance, an
equilibrium charge relaxation resistance and in the presence of transport
through one of the conductors a non-equilibrium charge relaxation resistance.
The discussion is illustrated for a quantum point contact in a high magnetic
field in proximity to a quantum dot.
