We present a model, based on the Keldysh formalism, to study the transport properties of disordered quantum wires of finite lengths. Unlike the phenomenological models, we estimate the electron in- and out-scattering rates using the local density of states of various transverse modes that allow electrons to change their directions. The formulation, therefore, guarantees the conservation of both the charge and the current at any cross section of the device. Using the model we calculate the average two-probe conductances of quantum wires that are terminated at nonideal contacts.