Hi all,

It was pointed out to me by Qingtian Zhang that the file that I posted
cannot actually be opened. There was some odd binary cruft at the
bottom. The non-mangled file is attached.

Happy kwanting,

# -*- coding: utf-8 -*-
import numpy as np

sigma_z = np.array([[1, 0], [0, -1]])

def spin_conductance(G, lead_out, lead_in, sigma=sigma_z):
    """Calculate the spin conductance between two leads.

    G : an instance of `kwant.solvers.common.GreensFunction`
        The Greens function of the system as returned by
    lead_out : integer
        The lead where spin current is collected
    lead_in : integer
        The lead where spin current is injected
    sigma : `numpy.ndarray` of shape (2, 2)
        The Pauli matrix of the quantization axis along
        which to measure the spin current

    Calculates the spin conductance between two leads
    p and q according to:

        G_{pq} =  Tr[ σ_{α} Γ_{q} G_{qp} Γ_{p} G^+_{qp} ]

    Where  Γ_{q} is the coupling matrix to lead q ( = i[Σ - Σ^+] )
    and G_{qp} is the submatrix of the system's Greens function
    between sites interfacing with lead p and q.
    ttdagger = G._a_ttdagger_a_inv(lead_out, lead_in)
    sigma_z_matrix = numpy.kron(np.eye(attdagger.shape[0]//2), sigma)
    return np.trace(sigma_z_matrix.dot(ttdagger)).real

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