where kF is the Fermi wave vector i

where kF is the Fermi wave vector in the region between the point contacts. In an experimental situation L and kF are known, so the collimation factor f can be directly determined from the collector voltage by means of Eq.(3.34).The result (3.34) holds in the absence of a magnetic field. A small magnetic field B will deflect the collimated electron beam past the collector. Simple geometry leads to the criterion L/2lcycl = αmax for the cyclotron radius at which Td is reduced to zero by the Lorentz force (assuming that L ≫ Wmax). One would thus expect to see in Vc/Ii a peak around zero field, of height given by Eq. (3.34) and of width