  We study the conductance properties of a straight two-dimensional quantum
wire with impurities modeled by $s$-like scatterers. Their presence can lead to
strong inter-channel coupling. It was shown that such systems depend
sensitively on the number of transverse modes included. Based on a poor man's
scaling technique we include the effect of higher modes in a renormalized
coupling constant. We therefore show that the low-energy behavior of the wire
is dominated by only a few modes, which hence is a way to reduce the necessary
computing power. The technique is successfully applied to the case of one and
two $s$-like scatterers.
