Hi Qingtian,
I suggest to put spin-up and spin-down on separate lattices, like in
http://kwant-project.org/doc/1.0/tutorial/tutorial5#lattice-description-using-different-lattices
This will split each lead into a spin-up and a spin-down lead and you
can easily calculate transmission between them.
Best,
Christoph
PS
Another, more general but also way more advanced solution would be to
get the wave functions of the lead modes, and project the spin current
operator on them. (Note that the object returned by kwant.smatrix
contains the wave functions of the modes:
http://kwant-project.org/doc/1.0/reference/generated/kwant.solvers.common.SMatrix)