A dewar wall is two vessels joined only at the neck, with the space between them evacuated and wrapped in reflective insulation. With almost no gas in that gap, heat cannot cross by conduction or convection, and radiation is cut by the foil layers. Which leaves one route in, and it is there on purpose.
Why the neck exists at all
It connects the inner and outer vessels and lets samples through. It is made as long and thin as the application allows, which is a straight trade against access.
The trade in practice
Every extra millimetre of neck diameter is a wider heat path and a higher evaporation rate. A vessel opened weekly can afford a narrow neck. One that racks are pulled from all day cannot. The CryoBank 1800-95 is listed with a 635 mm neck opening to take 100 cell racks, and accepts higher static boil off in exchange for that access.
What this means when choosing
Do not compare evaporation rates across vessels with different neck diameters as though they were the same product. Ask what the vessel is optimised for: throughput or hold time. Aluminium vessels such as the CryoCan 47-10, at 47 litres with ten canisters, sit at the accessible end of that trade.
