This is a back scabbard: http://www.by-the-sword.com/acatalog/OL1038.jpg
It is a baldric scabbard for traveling with the weapon, where the hanging strip is very short. The sword it is not useable this way. To be used in combat they'd take the scabbard off, take out the weapon and then do whatever with it. Not really a speed draw.
To those who say katana cannot be worn and drawn from back
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It is not drawn "from the back". This is a weird version of a baldric - a hanging scabbard. To be draw, the scabbard is pulled to the hip in the left hand and drawn from there like a normal sword.
Here is how a Nodachi is carried:
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loginEither like a big stick, or using the strap across the body - it dangles on the back, but drawn from the hip. It is not the fastest way to pull a sword, but at least it functions for a large blade. You can see it hanging here, starting at 3:15:
http://www.youtube.com/watch?v=Z5cyKZAbYQI&feature=player_detailpage#t=185sSeven Samurai is a great movie. The western version is not bad either though on a completely different dimension.
You can find wood-axe scabbard in shops to hand it from the back. Those I saw look like a pistol holster, the shaft goes through like the barrel and the head is covered in the leather like the pistol grip, so you do not hurt yourself. Again, it is not meant to be drawn in a hurry in case a tree attacks you - it is just a carrying device for travel to the tree.