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.