kwant.greens_function and kwant.smatrix give different results when computing reflection at energies with no open modes
AFAICT this is because GreensFunction._transmission computes the number of open modes by counting the nonzero eigenvalues of the broadening matrix Γ = i(Σ^R - Σ^A).
Eigenvalues of Γ are compared against 1000ε\Vert Γ \Vert where ε is floating point epsilon (\sim 10^{-15}), however when there are no open modes \Vert Γ \Vert \sim 10^{-15}. Eigenvalues are then compared against \sim 10^{-28}, and it looks like all the modes are open, even though there are 0 open.
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