A deposition system includes a process chamber for conducting an ALD process to deposit layers on a substrate. In one embodiment, instead of varying the gas flux on a substrate in the chamber by controlling the flow of gas upstream of the process chamber, the gas flux on the substrate is controlled by controlling the conductance between the process chamber and a lower pressure volume outside the process chamber. The flux of the gas on the substrate varies inversely with the chamber conductance, such that the flux of the gas on the substrate increases when the conductance decreases. Various methods of performing an ALD process by controlling the conductance are disclosed as well as various structures for controlling the conductance.