The collimation factor f ≥ 1 is the

The collimation factor f ≥ 1 is the product of a term describing the collimating effect of a barrier of height Ec (barrier collimation) and a term describing collimation due to a gradual widening of the point contact width from Wmin to Wmax (horn collimation). In the adiabatic approximation, the angular injection distribution P(α) is proportional to cosα with an abrupt truncation at ±αmax. The cosine angular dependence follows from the cosine distribution of the incident flux in combination with time-reversal symmetry and is thus not affected by the reduction of the injection/acceptance cone. We therefore conclude that in the adiabatic approximation P(α) (normalized to unity) is given by