Adiabatic electron collimation
= Adiabatic electron collimation
{title2=$p_\perp W\simeq\mathrm{constant}$}
Slow widening of a constriction approximately conserves the transverse mode action, reducing transverse momentum as the width $W$ increases. A lowering band bottom also increases longitudinal kinetic energy. Together they can narrow the angular distribution of an emitted electron beam. An abrupt narrow aperture alone can instead diffract strongly.