  The large-scale flow structure and the turbulent transfer of heat and
momentum are directly measured in highly turbulent liquid metal convection
experiments for Rayleigh numbers varied between $4 \times 10^5$ and $\leq 5
\times 10^9$ and Prandtl numbers of $0.025~\leq~Pr~\leq ~0.033$. Our
measurements are performed in two cylindrical samples of aspect ratios $\Gamma
=$ diameter/height $= 0.5$ and 1 filled with the eutectic alloy GaInSn. The
reconstruction of the three-dimensional flow pattern by 17 ultrasound Doppler
velocimetry sensors detecting the velocity profiles along their beamlines in
different planes reveals a clear breakdown of coherence of the large-scale
circulation for $\Gamma = 0.5$. As a consequence, the scaling laws for heat and
momentum transfer inherit a dependence on the aspect ratio. We show that this
breakdown of coherence is accompanied with a reduction of the Reynolds number
$Re$. The scaling exponent $\beta$ of the power law $Nu\propto Ra^{\beta}$
crosses \FIN{eventually} over from $\beta=0.221$ to 0.124 when the liquid metal
flow at $\Gamma=0.5$ reaches $Ra\gtrsim 2\times 10^8$ and the coherent
large-scale flow is completely collapsed.
