  Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic
fields are shown to exhibit dramatic changes in their band structure and
electron transport properties. A strong electric field across the ribbon
induces multiple chiral Dirac points, closing the semiconducting gap in
armchair GNR's. A perpendicular magnetic field induces partially formed Landau
levels as well as dispersive surface-bound states. Each of the applied fields
on its own preserves the even symmetry $E_{k} = E_{-k}$ of the subband
dispersion. When applied together, they reverse the dispersion parity to be odd
and gives $E_{e,k} = -E_{h,-k}$ and mix the electron and hole subbands within
the energy range corresponding to the change in potential across the ribbon.
This leads to oscillations of the ballistic conductance within this energy
range.
