Thanks for the reply.
I am calculating a real system, with 32 orbitals per unit cell. Moreover, I need to use supercells so that I can simulate periodical boundary conditions. For example, I want to scatter along x direction. Also, I want z direction to be finite and y direction to be periodical. Since kwant does not allow hoppings beyond nearest unit cell in leads, I have to make a supercell that contains all unit cells in y direction!
I want to calculate conductance between two different leads. This is what I have tried:
1. build an empty center area. This does not work because kwant doesn't know how to attach leads to an empty area.
2. use lead_1.attach_lead(lead_2). Kwant.plot does not correctly plot two leads, so I suppose it doesn't work.
I don't want to build a center area that is the same with one of the leads, because my system is very large and I suppose this will be time-consuming.