  Inconsistencies are pointed out in the usual quantum versions of the
classical linear Boltzmann equation constructed for a quantized test particle
in a gas. These are related to the incorrect formal treatment of momentum
decoherence. We prove that ideal collisions would result in complete momentum
decoherence, the persistence of coherence is only due to the finite
intercollision time. A corresponding novel quantum linear Boltzmann equation is
proposed.
