It was made for about a decade or more by Lothar-Walther. I had made many mentions about it here up until about five years ago.
It was fabricated by turning a barrel blank down to absolute minimum dimensions, then mating an aluminum sleeve onto it. The mating surfaces had to assure full contact for the full length. It was expensive, in the $700 range, available from 22 cal on up to 50 cal.
L-W Ultralight Barrel.
It transferred heat very quickly, and was attributed with exceptional accuracy. If I had the money, I'd own one now.
The listing disappeared when they reorganized their website a year or two ago. A vigorous search today failed to find any reference on their site. If you wish to delve deeper you can use their
contact info.
Greg