  It is demonstrated how the correction factor alpha used in Hall magnetometry
of localized magnetic field profiles depends on the sample geometry and on the
electron mean free path, in the quasi-ballistic and ballistic regimes, for weak
and strong magnetic field regimes. The frequently used approximation of a
constant correction factor close to 1 is generally not justified, especially in
the case of bipolar magnetic field profiles and may lead to large errors in the
determination of the magnitude of the magnetic fields. Rather, alpha depends in
a nontrivial way on the parameters of both the magnetic structure and the Hall
cross.
