ψ ψ is the digamma function. B ϕ ϕ is the phase coherence characteristic field, which is roughly the magnetic field required to destroy phase coherence, B SO is the spin–orbit characteristic field which can be considered a measure of the strength of the spin–orbit interaction and B e is the elastic characteristic field. The characteristic fields are better understood in terms of their corresponding characteristic lengths which are deduced from B i = ℏ ℏ / 4 e l i 2 . l ϕ ϕ can then be understood as the distance traveled by an electron before it loses phase coherence, l SO can be thought of as the distance traveled before the spin of the electron undergoes the effect of the spin–orbit interaction, and finally l e is the mean free path.