Thermally activated flow dynamics of polystyrene films supported by silicon is studied for a wide range of film thickness (h(0)) and molecular weights (M-w). At low M-w, the effective viscosity of the nanometer thin gins is smaller than the bulk and decreases With decreasing h(0). This is' due to enhancement Of the total Shear flow by the augmented mobility at the free surface. As M-w increases, with h(0) becoming smaller than the polymer radius of gyration (R-g), the effective viscosity switches froth being substrate-independent to substrate-dependent. We propose that interfacial slippage then dominates and leads to plug flow. The friction coefficient is found to increase with h(0) providing h(0)/R-g < similar to 1, demonstrating a surface-promoted confinement effect.