  Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building
blocks of semiconducting nanodevices, both exhibit famously anomalous
conductance features: the 0.7-anomaly in the former case, the Kondo effect in
the latter. For both the 0.7-anomaly and the Kondo effect, the conductance
shows a remarkably similar low-energy dependence on temperature $T$,
source-drain voltage $V_{\rm sd}$ and magnetic field $B$. In a recent
publication [F. Bauer et al., Nature, 501, 73 (2013)], we argued that the
reason for these similarities is that both a QPC and a KQD feature spin
fluctuations that are induced by the sample geometry, confined in a small
spatial regime, and enhanced by interactions. Here we further explore this
notion experimentally and theoretically by studying the geometric crossover
between a QD and a QPC, focussing on the $B$-field dependence of the
conductance. We introduce a one-dimensional model that reproduces the essential
features of the experiments, including a smooth transition between a Kondo QD
and a QPC with 0.7-anomaly. We find that in both cases the anomalously strong
negative magnetoconductance goes hand in hand with strongly enhanced local spin
fluctuations. Our experimental observations include, in addition to the Kondo
effect in a QD and the 0.7-anomaly in a QPC, Fano interference effects in a
regime of coexistence between QD and QPC physics, and Fabry-Perot-type
resonances on the conductance plateaus of a clean QPC. We argue that
Fabry-Perot-type resonances occur generically if the electrostatic potential of
the QPC generates a flatter-than-parabolic barrier top.
