We present calculations of the conductance in semiconductor quasi-one-dimensional systems using the Landauer formalism. We consider the effects on the transport properties inside semiconductor quantum waveguides of different shapes when a defect is located either in the wire region or in the quasi-two-dimensional region. We observe changes on the plateau's threshold when the defect is placed inside the wire and lowering of the conductance plateaus themselves below the conductance quantum G0 =2e² /h when the defect is outside the wire.