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