Boiling electrons off a hot filament constitutes a preparation providing a mixed ensemble with nearly Maxwellian distribution of momentum eigenstates. To further purify the ensemble, one can let the electrons enter an accelerating electric field, and then a transverse magnetic field. The desired momentum is then selected by filtering out all those undesired momenta having the wrong curvature in the magnetic field. Thus, it is possible to get a pure state of mometum say $p_0$.
This is the standard approach of most texbooks (including Heisenbergs in 1930) to explain the preparation of electrons in a pure momentum eigenstate (plane wave).
Question: Does someone know references in literature to real experiments for this kind of preparation and up to which precision the momentum of the plane wave can be obtained?