We measure the electron phase-coherence time up to 18 K using universal fluctuations in the lowtemperature magnetoconductance of an open InGaAs quantum dot. The temperature dependence of is quantitatively consistent with the two-dimensional model of electron-electron interactions in disordered systems. In our sample, is two to four times larger than previously reported in GaAs quantum dots. We attribute this enhancement to a larger value of the Fermi energy and the lower electron effective mass in our sample. We also observe a distinct type of conductance fluctuation due to ballistic electron focusing inside the dot up to 204 K.Since the pioneering work of Marcus and co-workers, 1 open ballistic quantum dots ͑QDs͒ have mainly been used as tools to investigate two main issues: ballistic electron dynamics and electron decoherence. When their mean free path becomes larger than the lateral dimensions of the QD, electrons can be viewed semiclassically as billiard balls. Depending on the geometrical symmetries of the QD, they can either follow stable periodic orbits, probe particular trajectories between the entrance and exit point contacts, or be reflected back into the entrance point contact. These classical ballistic effects can be predicted semiclassically or quantum mechanically and lead to large fluctuations of the magnetoconductance: 2-5 as the magnetic field is swept, the configuration of stable trajectories is modified, which affects the transmission through the QD.On the other hand, electron decoherence is mainly probed thanks to the emergence of two quantum corrections to the magnetoconductance, weak localization ͑WL͒ and universal conductance fluctuations ͑UCFs͒, arising from quantum interferences between trajectories inside the QD. As interferences only occur when phase coherence is maintained over the trajectory length, WL and UCFs give also access to the measurement of the electron phase-coherence time . [6][7][8][9] In GaAs/AlGaAs open QDs, it was shown 10,11 that the dephasing rate Ϫ1 is the sum of two electron-electron ͑e-e͒ scattering rates: a large energy-transfer scattering mechanism with a rate ee Ϫ1 , and a small energy-transfer ͑Nyquist͒ mechanism with a rate N Ϫ1 . The temperature dependence of Ϫ1 was found to be qualitatively consistent with the theoretical expression established for disordered two-dimensional ͑2D͒ electron systems: 10,12,13where E F is the Fermi energy, F is the Fermi wavelength, and l is the mean free path. However, a quantitative agreement with experimental data was only found for an arbitrary value of l , one order-of-magnitude smaller than l measured in the two-dimensional electron gas ͑2DEG͒. At very low temperatures, (T) deviates from the above temperature dependence and an unexplained saturation of is observed. 11,14 In the present work, we analyze magnetoconductance fluctuations arising from both phase-coherent and classical focusing effects in an InGaAs/InAlAs open circular QD over a wide temperature range ͑from 1.3 to 204 K͒. is extracted up to 18 K from the amplitu...

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