  We report on the measurement of phase coherence length in a high mobility
two-dimensional electron gas patterned in two different geometries, a wire and
a ring. The phase coherence length is extracted both from the weak localization
correction in long wires and from the amplitude of the Aharonov-Bohm
oscillations in a single ring, in a low temperature regime when decoherence is
dominated by electronic interactions. We show that these two measurements lead
to different phase coherence lengths, namely $L_{\Phi}^\mathrm{wire}\propto
T^{-1/3}$ and $L_{\Phi}^\mathrm{ring}\propto T^{-1/2}$. This difference
reflects the fact that the electrons winding around the ring necessarily
explore the whole sample (ergodic trajectories), while in a long wire the
electrons lose their phase coherence before reaching the edges of the sample
(diffusive regime).
