Take advantage of when a system is known to be Hermitian
Either
- omit the lead with most open channels by default when calling smatrix and greens_function or
- verify that the scattering matrix is indeed current-conserving (it might be not due to numerical error)
This may require rewriting solver interface, since usually users just call kwant.smatrix(sys)
without specifying the leads. Knowing in advance that only conductance is needed would be much better.