Sorry I only just saw this email; please remember to "reply All" so that the email gets sent to the mailing list too!
> Thanks. But except the PBC imposed on transverse direction, I need > attach the lead on longitude direction. So I want the transverse size > of sample to be finite. > Ah I see. In this case your could consider using 'kwant.wraparound' . In your case you would want to set the 'keep' paramter to '0' to keep the 0th translational symmetry in the wrapped around system. Using wraparound in this way will produce a Builder with a single (longitudinal) translational symmetry that you may use as a lead. The produced Builder will have an (additional) parameter 'k_y'; the transverse momentum. Don't be confused by the name; it will also work when the "wrapped" symmetry vector was not in the "y" direction! There were already several people on the mailing list who used wraparound, so you could also search the archives. If it's still not clear after this post back to this thread. Happy Kwanting, Joe : https://kwant-project.org/doc/1/reference/generated/kwant.wraparound.wraparound#kwant.wraparound.wraparound