A processing system is disclosed for conducting various processes on substrates, such as semiconductor wafers by varying the exposure to a chemical ambient. The processing system includes a processing region having an inlet and an outlet for flowing fluids through the chamber. The outlet is in communication with a conductance valve that is positioned in between the processing region outlet and a vacuum exhaust channel. The conductance valve rapidly oscillates or rotates between open and closed positions for controlling conductance through the processing region. This feature is coupled with the ability to rapidly pulse chemical species through the processing region while simultaneously controlling the pressure in the processing region. Of particular advantage, the conductance valve is capable of transitioning the processing region through pressure transitions of as great as 100:1 while chemical species are flowed through the processing region using equally fast control valves in a synchronous pulsed fashion.