  Through a series of transverse magnetic focusing experiments, we show that
hot electrons in a two-dimensional electron gas system undergo an ultrafast
relaxation when generated by a quantum dot (QD) instead of a quantum point
contact (QPC). We find here that QPC hot electrons were well described by the
non-interacting Fermi gas model for excitations up to 1.5 meV above the Fermi
level of 7.44 meV, whereas QD hot electrons exhibited an energy loss quadratic
to the excitation. The energy relaxation was a sizeable fraction of the tested
excitations, up to about 55%. With the proposal that the hot electrons are
relaxed by the QD immediately after emission, we present a toy model in which a
capacitive coupling between the QD and its leads results in a finite, ultrafast
energy relaxation.
