Absolutely poor chances of pregnancy if you have single tide fallopian tube because tube ligation is one of the most successful method of contraception with minimum chances of pregnancy. In case of inappropriate tube ligation or if tubes were tide few years back then chances of pregnancy are slightly high because as the time passes, there are chances that tubes become loose.
Even you have opened one tube then chances of pregnancy are at alternate months. Because in one month, one ovary is stimulated and egg is released. E.g. Suppose in the month of March, If right ovary is stimulated and egg is released from right ovary that goes in to right fallopian tube and in the month of April, left ovary will be stimulated and egg will be released from left ovary that will go in to left fallopian tube. If you don't have right fallopian tube or right fallopian tube is tied then you have no chances of pregnancy during this month. Your chances will be higher in the month of April.
Even you have opened one tube then chances of pregnancy are at alternate months. Because in one month, one ovary is stimulated and egg is released. E.g. Suppose in the month of March, If right ovary is stimulated and egg is released from right ovary that goes in to right fallopian tube and in the month of April, left ovary will be stimulated and egg will be released from left ovary that will go in to left fallopian tube. If you don't have right fallopian tube or right fallopian tube is tied then you have no chances of pregnancy during this month. Your chances will be higher in the month of April.