  The observation of coherent quantum transport phenomena in metals and
semiconductors is limited by the eventual loss of phase coherence of the
conducting electrons. We use the weak localization effect to measure the
low-temperature dephasing time in a two-dimensional electron Fermi liquid in
GaAs/AlGaAs heterostructures. We use a novel temperature calibration method
based on the integer quantum Hall effect in order to directly measure the
electrons' temperature. The data are in excellent agreement with recent
theoretical results, including contributions from the triplet channel, for a
broad temperature range. We see no evidence for saturation of the dephasing
time down to around 100mK. Moreover, the zero-temperature dephasing time is
extrapolated to be higher than 4ns.
