`lead.cell_hamiltonian()` gives 32000. The thickness of my slab is 50. Is this something not possible to calculate?
Ok, now it certainly makes sense that the memory blows up; the kwant paper indicates that the memory requirement for solving this sort of size should be ~ 100GB.
Anton reminded me of a workaround written by Christoph for dealing with periodic systems. You can find the code over here (if you don't want to download it using git you can get the raw python module here). You should be able to use this instead of replicating your unit cell in the y direction, which should drastically reduce your memory/runtime requirements.