  We demonstrate that branching of the electron flow in semiconductor
nanostructures can strongly affect macroscopic transport quantities and can
significantly change their dependence on external parameters compared to the
ideal ballistic case even when the system size is much smaller than the mean
free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs
two-dimensional electron gas we observe a splitting of the commensurability
peaks in the magnetoresistance curve. We show that a model which includes a
random disorder potential of the two-dimensional electron gas can account for
the random splitting of the peaks that result from the collimation of the
electron beam. The shape of the splitting depends on the particular realization
of the disorder potential. At the same time magnetic focusing peaks are largely
unaffected by the disorder potential.
