  Scattering by magnetic impurities is known to destroy coherence of electron
motion in metals and semiconductors. We investigate the decoherence introduced
in a single act of electron scattering by a magnetic impurity in a quantum Hall
system. To this end we introduce a fictitious nonunitary scattering matrix
$\mathcal{S}$ for electrons that reproduces the exactly calculated scattering
probabilities. The strength of decoherence is identified by the deviation of
eigenvalues of the product $\mathcal{S}\mathcal{S}^{\dagger}$ from unity. Using
the fictitious scattering matrix, we estimate the width of the metallic region
at the quantum Hall effect inter-plateau transition and its dependence on the
exchange coupling strength and the degree of polarization of magnetic
impurities.
