shubnikov de haas oscillations, conductance quantization

Serge87

Hi all
I'm just studying the QHE and Shubnikov de Haas oscillations. There are two points I find somehow confusing:

1. If you look at ρ_xx (resistance along the direction of applied field), you will find oscillations of this resistance as a function of the external magnetic field. Whenever the Fermi level lies in between 2 landau levels, there is no scattering and therefore ρ_xx is zero. But on the other hand, each edge channel has a finite amount of conductance (2e²/h). So how can it be, that resistance is 0 while conductance is not infinite?

2. The edge channel model is scetched as follows:


What I dont get here: Why can you measure a current at all in this situation? I mean, on the left edge the current is running down, while on the right edge current is running up? So from this point of view there is no net current? Or is it, since the Lorentz force is pushing the electrons to one of the two edges, there is an unequal number of edge channels on both sides?

I hope my questions are clear (I'm not native english speaker)

would be great to get some inputs here.
Thanks in advance
cheers
Serge