  We review the first experiment on dynamic transport in a phase-coherent
quantum conductor. In our discussion, we highlight the use of time-dependent
transport as a means of gaining insight into charge relaxation on a mesoscopic
scale. For this purpose, we studied the ac conductance of a model quantum
conductor, i.e. the quantum RC circuit. Prior to our experimental work, M.
B\"{u}ttiker, H. Thomas and A. Pr\^{e}tre first worked on dynamic mesoscopic
transport in the 1990s. They predicted that the mesoscopic RC circuit can be
described by a quantum capacitance related to the density of states in the
capacitor and a constant charge relaxation resistance value equal to half of
the resistance quantum h/2e^2, when a single mode is transmitted between the
capacitance and a reservoir. By applying a microwave excitation to a gate
located on top of a coherent submicronic quantum dot that is coupled to a
reservoir, we validate this theoretical prediction on the ac conductance of the
quantum RC circuit. Our study demonstrates that the ac conductance is directly
related to the dwell time of electrons in the capacitor. Thereby, we observed a
counterintuitive behavior of a quantum origin: as the transmission of the
single conducting mode decreases, the resistance of the quantum RC circuit
remains constant while the capacitance oscillates.
