We use two simple models, a phase accumulation model for quantum well (QW) resonances and a two-current circuit model, to study electron and spin transport in nanoscale systems consisting of barrier-conductor-barrier structures. Such structures include double barrier tunnel junctions, and long chain molecules with weak contacts to two electrodes. The phase accumulation model is applied to QW states in ultrathin films and nanodots sandwiched between two barriers. This model is used to illustrate the difference in the QW confinement effect between thin films and nanodots, and justifies circuit model which describes sequential tunneling. The circuit model for spin transport leads to a generalization of the well-known Julliere's formula and accounts for the conductivity mismatch. The circuit model is also applied to organic light-emitting diodes and yields a simple formula for the quantum efficiency. The results yield excellent agreement with experiment. (C) 2007 Elsevier Ltd. All rights reserved.