We report on an investigation into confined geometry effects and critical currents of superfluid3He in a single circular cylindrical channel. The diameter of the channel, 0.7 μm, is of the order of the (temperature-dependent) coherence length and its aspect ratio is ∼10. The reduction of the critical temperature demonstrates diffuse scattering on the solid walls of the microchannel. Using the Ginzburg-Landau formulation, we derive a model for the critical current and the critical temperature in a small, infinitely long, cylindrical channel with a circular cross section. The measured reductions of these quantities are in reasonable agreement with the predictions of the model.