  We report low-temperature electronic transport measurements performed in two
multi-terminal Corbino samples formed in GaAs/Al-GaAs two-dimensional electron
gases (2DEG) with both ultra-high electron mobility ($\gtrsim 20\times 10^6$
$cm^2/Vs)$ and with distinct electron density of $1.7$ and $3.6\times
10^{11}~cm^{-2}$. In both Corbino samples, a non-monotonic behavior is observed
in the temperature dependence of the resistance below 1~$K$. Surprisingly, a
sharp {\it decrease} in resistance is observed with {\it increasing}
temperature in the sample with lower electron density, whereas an opposite
behavior is observed in the sample with higher density. To investigate further,
transport measurements were performed in large van der Pauw samples having
identical heterostructures, and as expected they exhibit resistivity that is
monotonic with temperature. Finally, we discuss the results in terms of various
lengthscales leading to ballistic and hydrodynamic electronic transport, as
well as a possible Gurzhi effect.
