  Physical self-adjoint extensions and their spectra of the one-dimensional
Dirac type Hamiltonian operator in which both the mass and velocity are
constant except for a finite jump at one point of the real axis are correctly
found. Different boundary conditions on envelope wave functions are studied,
and the limiting case of equal masses (with no mass jump) is reviewed.
Transport across one-dimensional heterostructures described by the Dirac
equation is considered.
