¼ barspoon of xanthan gum?
If you are lucky enough, like me, to own an Easy Jigger, and the author was referring to an 'American barspoon,' then you can measure 1/24 ounce.

If a Barspoon = 5 ml
then ¼ Barspoon = 1.25 ml = 0.042 oz (approx 1/24 oz)

If a Barspoon = 2.5 ml
then ¼ Barspoon = 0.625 ml = 0.0211 oz (approx 1/48 oz)

If a Barspoon = 2.4 ml = 1/8 oz
then ¼ Barspoon = 0.6 ml = 0.02 oz (approx 1/50 oz)