By solving the Bogoliubov-de Gennes equation self-consistently, we compute transport properties of various normal-superconducting mesoscopic structures. Three structures are considered; a single superconducting island, a superconducting island with a delta-function scatterer at one interface and a superconducting island with a delta-function scatterer at the centre of the island. The calculated self-consistent I-V characteristics show significant structure, arising from the competition between scattering processes at the boundaries of the island and modification of the order parameter by quasi-particles and superflow. When the order parameter is significantly modified by the transport current, it is noted that the magnitude of the differential conductance of a single-channel conductor can exceed 2e(2)/h.