  Resistively-detected NMR (RDNMR) is a unique characterization method enabling
highly-sensitive NMR detection for a single quantum nanostructure, such as a
quantum point contact (QPC). In many studies, we use dynamic nuclear
polarization and RDNMR detection in a quantum Hall breakdown regime of a local
QPC filling factor of 1 ($\nu_{\rm{qpc}} = 1$). However, the lineshapes of
RDNMR signal are proved to be complicated and still not fully understood yet.
Here, we systematically polarize the nuclear spins by current-pumping from the
close vicinity of $\nu_{\rm{qpc}} = 1$ conductance plateau all the way down to
pinch-off point, providing a clear evidence that the spin-flip scattering
between two edge channels at the lowest Landau level still occurs in the
constriction even when it is close to the pinch off point ($G \approx 10^{-2}$
$2e^{2}/h$). The collected RDNMR spectra reveal two sets of distinguished
features. First, in a strong to intermediate tunneling regime, we observe an
ordinary resistance dip lineshape but interestingly its transition frequency
follows a snake-like pattern, an indicative of spatial modulation of electron
density in the QPC. Second, in a weak tunneling regime, the spectrum turns into
a dispersive lineshape which we interpret due to the build up of two sets of
nuclear spin polarization that are in contact with different electron spin
polarization.
