  Coherent electron transport through a quantum channel in the presence of a
general extended scattering potential is investigated using a T-matrix
Lippmann-Schwinger approach. The formalism is applied to a quantum wire with
Gaussian type scattering potentials, which can be used to model a single
impurity, a quantum dot or more complicated structures in the wire. The well
known dips in the conductance in the presence of attractive impurities is
reproduced. A resonant transmission peak in the conductance is seen as the
energy of the incident electron coincides with an energy level in the quantum
dot. The conductance through a quantum wire in the presence of an asymmetric
potential are also shown. In the case of a narrow potential parallel to the
wire we find that two dips appear in the same subband which we ascribe to two
quasi bound states originating from the next evanescent mode.
