  We report thermopower ($S$) and electrical resistivity ($\rho_{2DES}$)
measurements in low-density (10$^{14}$ m$^{-2}$), mesoscopic two-dimensional
electron systems (2DESs) in GaAs/AlGaAs heterostructures at sub-Kelvin
temperatures. We observe at temperatures $\lesssim$ 0.7 K a linearly growing
$S$ as a function of temperature indicating metal-like behaviour. Interestingly
this metallicity is not Drude-like, showing several unusual characteristics: i)
the magnitude of $S$ exceeds the Mott prediction valid for non-interacting
metallic 2DESs at similar carrier densities by over two orders of magnitude;
and ii) $\rho_{2DES}$ in this regime is two orders of magnitude greater than
the quantum of resistance $h/e^2$ and shows very little temperature-dependence.
We provide evidence suggesting that these observations arise due to the
formation of novel quasiparticles in the 2DES that are not electron-like.
Finally, $\rho_{2DES}$ and $S$ show an intriguing decoupling in their
density-dependence, the latter showing striking oscillations and even sign
changes that are completely absent in the resistivity.
