  Propagation of high-intensity, high-contrast ($<10^{-8}$), 50\,fs laser
pulses through triangilar copper capillaries is experimentally studied. The
relative transmission through 20-mm-long, about 50\,$\mu$m wide capillaries is
directly measured to be 70\% for input intensities up to
$10^{17}\,\text{W/cm}^2$. The copper reflectivity in vacuum, helium, and air is
measured in the intensity range of $10^{10}$--$10^{17}\,\text{W/cm}^2$. No
reflectivity decrease in vacuum and helium is observed, which leads to the
conclusion that copper capillary waveguides can efficiently guide laser pulses
of intensities up to $10^{19}\,\text{W/cm}^2$ on the capillary axis (that
corresponds to $10^{17}\,\text{W/cm}^2$ on the walls). The reduction of the
transmission efficiency to zero after a number of transmitted pulses is
observed, which is caused by plug formation inside the capillary. The
dependence of the capillary lifetime on the pulse energy is measured.
