  Many strongly correlated transition metal oxides exhibit a metal-insulator
transition (MIT), the manipulation of which is essential for their application
as active device elements. However, such manipulation is hindered by lack of
microscopic understanding of mechanisms involved in these transitions. A
prototypical example is VO2, where previous studies indicated that the MIT
resistance change correlate with changes in carrier density and mobility. We
studied the MIT using Hall measurements with unprecedented resolution and
accuracy, simultaneously with resistance measurements. Contrast to prior
reports, we find that the MIT is not correlated with a change in mobility, but
rather, is a macroscopic manifestation of the spatial phase separation which
accompanies the MIT. Our results demonstrate that, surprisingly, properties of
the nano-scale spatially-separated metallic and semiconducting domains actually
retain their bulk properties. This study highlights the importance of taking
into account local fluctuations and correlations when interpreting transport
measurements in highly correlated systems.
