  We consider the weak localization in a ring connected to reservoirs through
leads of finite length and submitted to a magnetic field. The effect of
decoherence due to electron-electron interaction on the harmonics of AAS
oscillations is studied, and more specifically the effect of the leads. Two
results are obtained for short and long leads regimes. The scale at which the
crossover occurs is discussed. The long leads regime is shown to be more
realistic experimentally.
