  We report measurements of the electron dephasing time extracted from the weak
localization (WL) correction to the average conductance in an open AlGaAs/GaAs
quantum dot from 1 K to 13 mK. In agreement with theoretical predictions but in
contrast with previous measurements in quantum dots, the extracted dephasing
time does not saturate at the lowest temperatures. We find that the dephasing
time follows an inverse linear power law with temperature. We determine that
the extraction of the dephasing time from WL is applicable down to our lowest
temperatures, but extraction from finite magnetic field conductance
fluctuations is complicated by charging effects below 13 mK.
