  We consider the two-dimensional chiral metal, which exists at the surface of
a layered, three-dimensional sample exhibiting the integer quantum Hall effect.
We calculate its magnetoresistance in response to a component of magnetic field
perpendicular to the sample surface, in the low temperature, but macroscopic,
regime where inelastic scattering may be neglected. The magnetoresistance is
positive, following a Drude form with a field scale,
$B_0=\Phi_0/al_{\text{el}}$, given by the transverse field strength at which
one quantum of flux, $\Phi_0$, passes through a rectangle with sides set by the
layer-spacing, $a$, and the elastic mean free path, $l_{\text{el}}$.
Experimental measurement of this magnetoresistance may therefore provide a
direct determination of the elastic mean free path in the chiral metal.
