Soba is a grain crop known as 'buckwheat' in English. It is best known for the noodles that it is made into, also known as soba. Soba is one of those words for which there was no suitable Chinese character available, only the compound word qiao3mai4. As a result, the word soba is written in Chinese characters. While seldom seen, this sometimes appears on menus, as on the following menu outside an upmarket restaurant in Tokyo.
For the most part, however, soba is written in hiragana, as in the following sign.
A Google search revealed the following pattern of usage (August 2003):