Metallic nanowires are of great interest as interconnects in future nanoelectronic circuits. They also represent important systems for understanding the complexity of electronic interactions and conductivity in one-dimension. We have fabricated exceptionally long and uniform YSi 2 nanowires via self-assembly of yttrium atoms on Si(001). The thinnest wires represent one of the closest realizations of the isolated Peierls chain, exhibiting van-Hove type singularities in the onedimensional density of states and charge order fluctuations below 150 K. The structure of the wire was determined though a detailed comparison of scanning tunneling microscopy data and firstprinciples calculations. Sporadic broadenings of the wires' cross section imply the existence of a novel metal-semiconductor junction whose electronic properties are governed by the finite-sizeand temperature-scaling of the charge ordering correlation.One-dimensional (1D) conductors have always captured the imagination of physicists. While a strictly 1D material remains a theoretical construct, a vast number of materials can be viewed as quasi 1D, making them interesting test cases for theoretical predictions and nanoscale applications. Classic examples include charge transfer complexes such as TTF-TCNQ and the Bechgaard salts, which have been studied extensively because of their unusual electrical properties and potential applications as organic conductors (1). These compounds can be viewed as macroscopic ensembles of weakly-coupled quantum chains. They are also textbook illustrations of the venerable Peierls theorem, which states that a 1D metallic chain should be unstable with respect to a symmetry-lowering modulation of the atomic coordinates and valence-charge density (2). The mechanism driving this instability entails the strong coupling between the vibrational modes of the lattice and electrons near the Fermi level. The ground state of a Peierls chain is insulating but the band gap decreases with increasing temperature and metallicity is ultimately restored via a second order phase transition.This simple picture contains some interesting caveats. First of all, the Peierls theory ignores thermodynamic fluctuations. In the 1D limit, thermal fluctuations always destroy long-range order so in principle there is no phase transition (3). Paradoxically, 2D or 3D interchain coupling appears essential for observing the Peierls phase transition in real systems (3). Secondly, the Peierls theory completely ignores electron-electron correlations. Depending on the various interaction parameters, the Peierls transition may be preempted by a competing 'many-body' ground state such as a spin density wave state or Luttinger liquid (1-3). To date, the closest experimental realization of a 1D model system is, arguably, the single-wall carbon nanotube (4-7). Yet, there is no experimental evidence of a Peierls distortion in nanotubes, possibly because the tube diameters are not small enough for observing the transition (5-7). Some experiments even suggest the ex...