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The semiconductor quantum point contact has long been a focal point forstudies of one-dimensional electron transport. Their electrical properties aretypically studied using ac conductance methods, but recent work has shown thatthe dc conductance can be used to obtain additional information, with adensity-dependent Land\'{e} effective g-factor recently reported [T.-M. Chen etal, Phys. Rev. B 79, 081301 (2009)]. We discuss previous dc conductancemeasurements of quantum point contacts, demonstrating how valuable additionalinformation can be extracted from the data. We provide a comprehensive andgeneral framework for dc conductance measurements that provides a path toimproving the accuracy of existing data and obtaining useful additional data. Akey aspect is that dc conductance measurements can be used to map the energy ofthe 1D subband edges directly, giving new insight into the physics that takesplace as the spin-split 1D subbands populate. Through a re-analysis of the dataobtained by Chen et al, we obtain two findings. The first is that the 2-downsubband edge closely tracks the source chemical potential when it first beginspopulating before dropping more rapidly in energy. The second is that the 2-upsubband populates more rapidly as the subband edge approaches the drainpotential. This second finding suggests that the spin-gap may stop opening, oreven begin to close again, as the 2-up subband continues populating, consistentwith recent theoretical calculations and experimental studies.

Tracking the energies of one-dimensional subband edges in quantum point contacts using dc conductance measurements